The present invention relates to processes and materials for aligning liquid crystals, and liquid crystal optical elements.
Current liquid crystal display (LCD) elements include a product that utilize a twisted nematic mode, i.e., having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 90.degree. between a pair of upper and lower electrode substrates, a product utilizing a supertwisted nematic mode, utilizing a birefringent effect, i.e. having a structure wherein the aligning direction of nematic liquid crystal molecules is twisted by 180.degree. to 300.degree., an in-plane-switching mode wherein both electrodes controlling the liquid crystal alignment are present on one substrate and the direction of the liquid crystal orientation in the plane of the substrate changes upon application of an electric field, and a product utilizing a ferroelectric liquid crystal substance or an antiferroelectric liquid crystal substance. Common to each of these products is a liquid crystal layer disposed between a pair of substrates coated with a polymeric alignment layer. The polymeric alignment layer controls the direction of alignment of the liquid crystal medium in the absence of an electric field. Usually the direction of alignment of the liquid crystal medium is established in a mechanical buffing process wherein the polymer layer is buffed with a cloth or other fiberous material. The liquid crystal medium contacting the buffed surface typically aligns parallel to the mechanical buffing direction. Alternatively, an alignment layer comprising anisotropically absorbing molecules can be exposed to polarized light to align a liquid crystal medium as disclosed in U.S. Pat. No. 5,807,498 "Process and Materials for Aligning Liquid Crystals and Liquid Crystal Optical Elements".
The process for aligning liquid crystal media with polarized light is a non-contact method of alignment that has the potential to reduce dust and static charge buildup on alignment layers. Other advantages of the optical alignment process include high resolution control of alignment direction and high quality of alignment.
Requirements of optical alignment layers for liquid crystal displays include low energy threshold for alignment, transparency to visible light (no color), good dielectric properties and voltage holding ratios (VHR), long-term thermal and optical stability, and in many applications a controlled uniform pre-tilt angle.
Most liquid crystal devices, including displays, have a finite pre-tilt angle, controlled, for instance, by the mechanical buffing of selected polymeric alignment layers. The liquid crystal molecules in contact with such a layer aligns parallel to the buffing direction, but is not exactly parallel to the substrate. The liquid crystal molecules are slightly tilted from the substrate, for instance by about 2-15 degrees. For optimum performance in most display applications a finite and uniform pre-tilt angle of the liquid crystal is desirable.
Polymers used in forming optical alignment layers also must have a reasonably broad processing window. Polymers used as alignment layers in commercial liquid crystal displays are generally polyimide-based systems because of their good thermal and electrical properties. Thus, within the polyimide family, polymers also must have functionality that is stable to thermal and/or chemical imidization. In addition, polymers must have good wetting characteristics and printability onto substrates to give uniform layers.
Several approaches have been explored to meet the performance requirements of optical alignment layers for liquid crystal displays. In particular, U.S. Pat. No. 5,807,498 describes polyimide optical alignment layers having diaryl ketones as the anisotropically absorbing molecules. These materials can give good to excellent uniformity of alignment of liquid crystals. However, mass production of liquid crystal displays generally requires materials that are more photosensitive to light than the diaryl ketone based polyimides, and have improved electrical properties, especially with regard to VHR.
In further developing materials and processes for optical alignment layers, new classes of reactive materials have been developed that suggest an increased photosensitivity of polyimides by incorporating higher densities of anisotropically absorbing moieties. Surprisingly, the new materials exhibit improved VHR, especially at elevated temperatures.